Method and apparatus for performing mobile assisted hard handoff between communication systems

ABSTRACT

A method and apparatus for performing inter-system hard handoff between communication systems or inter-frequency hard handoff within a CDMA communication system is disclosed. The purpose of this invention is to reduce the probability of dropped calls during inter-system hard handoff. In the event that a hard handoff attempt is unsuccessful, the mobile station will return to the original system with information which the communication system of the present invention uses to assist in the performance of future handoff attempts. Alternatively, with no handoff attempt made, the mobile station monitors the destination system then returns to the original system with information used to assist in subsequent handoff attempts. The information returned from monitoring a CDMA system consists of results of a search for one or more pilots given at offsets in a specific list provided to the mobile station by the base station or a set of offsets based upon a predetermined search algorithm.

This is a continuation of application Ser. No. 08/784,280, filed Jan.15, 1997, now U.S. Pat. No. 5,940,761.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to communications systems. Moreparticularly, the present invention relates to a novel and improvedmethod for hard handoff between different wireless communicationsystems.

II. Description of the Related Art

In a code division multiple access (CDMA) spread spectrum communicationsystem, a common frequency band is used for communication with all basestations within that system. An example of such a system is described inthe TIA/EIA Interim Standard IS-95-A entitled “Mobile Station-BaseStation Compatibility Standard for Dual-Mode Wideband Spread SpectrumCellular System”, incorporated herein by reference. The generation andreceipt of CDMA signals is disclosed in U.S. Pat. No. 4,401,307 entitled“SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMS USING SATELLITEOR TERRESTRIAL REPEATERS” and in U.S. Pat. No. 5,103,459 entitled“SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONESYSTEM” both of which are assigned to the assignees of the presentinvention and incorporated herein by reference.

Signals occupying the common frequency band are discriminated at thereceiving station through the spread spectrum CDMA waveform propertiesbased on the use of a high rate pseudonoise (PN) code. A PN code is usedto modulate signals transmitted from the base stations and the remotestations. Signals from different base stations can be separatelyreceived at the receiving station by discrimination of the unique timeoffset that is introduced in the PN codes assigned to each base station.The high rate PN modulation also allows the receiving station to receivea signal from a single transmission station where the signal hastraveled over distinct propagation paths. Demodulation of multiplesignals is disclosed in U.S. Pat. No. 5,490,165 entitled “DEMODULATIONELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS”and in U.S. Pat. No. 5,109,390 entitled “DIVERSITY RECEIVER IN A CDMACELLULAR TELEPHONE SYSTEM”, both of which are assigned to the assigneeof the present invention and incorporated herein by reference.

The common frequency band allows simultaneous communication between aremote station and more than one base station, a condition known as softhandoff disclosed in U.S. Pat. No. 5,101,501 entitled “SOFT HANDOFF IN ACDMA CELLULAR TELEPHONE SYSTEM” and U.S. Patent No. 5,267,261 entitled“MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONSSYSTEM”, both assigned to the assignee of the present invention andincorporated herein by reference. Similarly, a remote station can besimultaneously communicating with two sectors of the same base station,known as softer handoff as disclosed in U.S. Pat. No. 5,625,876 entitled“METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMONBASE STATION”, issued Apr. 29, 1997, assigned to the assignee of thepresent invention and incorporated herein by reference. Handoffs aredescribed as soft and softer because they make the new connection beforebreaking the existing one.

If a mobile station travels outside the boundary of the system withwhich it is currently communicating, it is desirable to maintain thecommunication link by transferring the call to a neighboring system, ifone exists. The neighboring system may use any wireless technology,examples of which are CDMA, NAMPS, AMPS, TDMA or FDMA. If theneighboring system uses CDMA on the same frequency band as the currentsystem, an inter-system soft handoff can be performed. In situationswhere intersystem soft handoff is not available, the communication linkis transferred through a hard handoff where the current connection isbroken before a new one is made. Examples of hard handoffs are thosefrom a CDMA system to a system employing an alternate technology or acall transferred between two CDMA systems which use different frequencybands (inter-frequency hard handoff).

Inter-frequency hard handoffs can also occur within a CDMA system. Forexample, a region of high demand such as a downtown area may require agreater number of frequencies to service demand than the suburban regionsurrounding it. It may not be cost effective to deploy all availablefrequencies throughout the system. A call originating on a frequencydeployed only in the high congestion area must be handed off as the usertravels to a less congested area. Another example is that of a microwaveor other service operating on a frequency within the system'sboundaries. As users travel into an area suffering from interferencefrom the other service, their call may need to be handed off to adifferent frequency.

Handoffs can be initiated using a variety of techniques. Handofftechniques, including those using signal quality measurements toinitiate handoff, are found in U.S. Pat. No. 5,697,055 entitled “METHODAND APPARATUS FOR HANDOFF BETWEEN DIFFERENT CELLULAR COMMUNICATIONSSYSTEMS”, issued Dec. 9, 1997, assigned to the assignee of the presentinvention and incorporated herein by reference. Further disclosure onhandoffs, including measurement of round-trip signal delay to initiatehandoff, is disclosed in U.S. Pat. No. 5,848,063 entitled “METHOD ANDAPPARATUS FOR HARD HANDOFF IN A CDMA SYSTEM”, issued Dec. 8, 1998,assigned to the assignee of the present invention and incorporatedherein by reference. Handoffs from CDMA systems to alternate technologysystems are disclosed in U.S. Pat. No. 5,594,718 ('718 patent) entitled“METHOD AND APPARATUS FOR MOBILE UNIT ASSISTED CDMA TO ALTERNATIVESYSTEM HARD HANDOFF”, issued Jan. 14, 1997, assigned to the assignee ofthe present invention and incorporated herein by reference. In the '708patent, pilot beacons are placed at the boundaries of the system. When amobile station reports these pilots to the base station, the basestation knows that the mobile station is approaching the boundary.

When a system has determined that a call should be transferred toanother system via hard handoff, a message is sent to the mobile stationdirecting it to do so along with parameters that enable the mobilestation to connect with the destination system. The system has onlyestimates of the mobile station's actual location and environment, sothe parameters sent to the mobile station are not guaranteed to beaccurate. For example, with beacon aided handoff, the measurement of thepilot beacon's signal strength can be a valid criteria for triggeringthe handoff. However, the appropriate cell or cells in the destinationsystem which are to be assigned to the mobile station (known as theActive Set) are not necessarily known. Moreover, including all thelikely possibilities may exceed the maximum allowable in the Active Set.

In order for the mobile station to communicate with the destinationsystem, it must lose contact with the old system. If the parametersgiven to the mobile station are not valid for any reason, i.e. changesin the mobile station's environment or lack of precise locationinformation at the base station, the new communication link will not beformed, and the call may be dropped. After an unsuccessful handoffattempt, the mobile station can revert back to the previous system if itis still possible to do so. With no further information and nosignificant change in the mobile station's environment, repeatedattempts to hand off will also fail. Thus, there is a need felt in theart for a method for performing additional hard handoff attempts withgreater probability of success.

SUMMARY OF THE INVENTION

The purpose of this invention is to reduce the probability of droppedcalls during inter-system hard handoff. In the event that a hard handoffattempt is unsuccessful, the mobile station will return to the originalsystem with information which the communication system of the presentinvention uses to assist in the performance of future handoff attempts.

Prior to handoff, the original base station will have a rough estimateof the most likely base stations of a destination system to provideservice to a mobile station as it travels into the destination system.In the exemplary embodiment, a message will be sent from the basestation to the mobile station containing this list of neighboring basestations in the destination system, a minimum total received powerthreshold, and a minimum pilot energy threshold. When the base stationin the original system has determined that a hard handoff isappropriate, it signals the neighboring base stations in the destinationsystem to begin transmitting forward link traffic to the mobile stationentering the system. A first hard handoff is attempted after a messageis received by the mobile station from the base station initiating theinter-system hard handoff. The mobile station switches to the frequencyof the destination system and attempts to acquire the base stations ofthe destination system in accordance with the acquisition parametersprovided (i.e. the pilot PN offsets). If the minimum pilot energythreshold is exceeded, the handoff is deemed to be successful and themobile station remains on the destination system.

If the minimum pilot energy threshold is not exceeded, recoverytechniques begin. The mobile station measures the total in-band energyof the destination system and compares that to the total received powerthreshold. If the minimum total received power threshold is notexceeded, the handoff is abandoned immediately. The mobile stationreturns to the original system and reports that no significant power wasdetected at the new frequency. If the minimum total received power isexceeded, it is likely that the destination system is available, butthat the neighboring base stations provided by the original system(referred to as the new Active Set) is not acceptable for communication.The mobile station then performs a search to locate viable pilot signalsin the destination system. In general, a list of offsets to searchprovided to the mobile station will be sufficient to locate availablepilots, although other search algorithms can be employed. Uponcompletion of the search, the mobile station returns to the originalsystem and reports the failure and any pilot signals found in the searchwhich exceeded a third threshold.

If no significant received power was detected or there were no pilotsfound in the search, the system controller can opt to delay a secondattempt at handoff in hopes of a beneficial change in the mobilestation's environment. In the alternative, the mobile station canabandon the hard handoff attempt altogether, which would likely resultin the eventual dropping of the call. However, in those cases where thedestination system is present, the system controller can update theActive Set based on the returned search information, and the destinationsystem can modify the base stations transmitting to the mobile stationaccordingly. Then a second hard handoff attempt message can be sent tothe mobile station. Unless the environment has changed, this secondattempt is likely to be successful.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 is a schematic overview of an exemplary spread spectrum CDMAcommunications system in accordance with the present invention;

FIG. 2 is a representation of example scenarios whereby the varioussituations responded to by this invention can be described;

FIG. 3 is an illustration of an exemplary base station;

FIG. 4 is an illustration of an exemplary mobile station; and

FIG. 5 is a flow chart illustrating the operation of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 depicts an embodiment of a communication system employing thepresent invention. A typical CDMA communications system consists of asystem controller and switch 10 in communication with one or more basestations, examples of which are 12, 14, and 16. System controller andswitch 10 also connects with the Public Switched Telephone Network(PSTN) (not shown) and with other communication systems (not shown).Mobile station 18 is an example subscriber with forward links 20B, 22B,and 24B, and reverse links 20A, 22A, and 24A. The system controller andswitch 10 controls soft handoffs and inter-frequency hard handoffswithin the system, and in conjunction with neighboring systems controlsinter-system soft handoff as well as inter-system hard handoffs. Theexemplary embodiment of the present invention deals with CDMA system toCDMA system interfrequency hard handoffs. It will be understood by oneskilled in the art that the teachings of the present invention can beapplied to handoffs using multiple access schemes and for handoffbetween systems using different modulation schemes.

FIG. 2 depicts three different scenarios possible in the use of thepresent invention. Three mobile stations, M1, M2, and M3 are travelingfrom the system where their respective calls originated, S1, to aneighboring system of differing frequency, S2. Initially, all the mobilestations M1-M3 are in communication with one or more base stations (notshown) in system S1. As each mobile station travels across the boundaryof S1 into S2, a hard handoff attempt will be made. The destinationsystem, S2, contains base stations B1-B5 each which cover a cell areaC1-C5, respectively. System S2 may have other base stations (not shown)which do not affect the scenarios given. As shown, some cells intersectwith other cells. In that overlapping region, a mobile station can be incommunication with either base station or both simultaneously if themobile station is in soft handoff. Also shown are obstructions 01-03.These obstructions distort the coverage areas that otherwise would becircular shaped cells. Cell C5 is shown shaded to clearly indicate itsunusual shape.

Consider first mobile station M1. This is an example of a case whichwould result in a successful hard handoff in both the state of the artand the current invention. As M1 approaches the S1-S2 border,origination system S1 predicts the likely neighbors in destinationsystem S2, based on its best guess of the location of M1. S1, through abase station in contact with M1 (not shown) then notifies M1 of the PNoffsets of cells in the destination system S2, for example C1, C2, C3,C4, and C5. In the exemplary embodiment, S2 also sends parameters forminimum total received pilot, MIN_TOT_PILOT, and minimum received power,MIN_RX_PWR. In an alternative embodiment, M1 may store values ofMIN_TOT_PILOT and MIN_RX_PWR or may be capable of generating the valuesbased on system data. S1 then begins forwarding traffic to system S2with instructions to set up the appropriate forward link for that datadirected to mobile station M1 on base stations B2 and B3. Base stationsB2 and B3 are the most likely target base stations and are in the newActive Set. Then S1 sends an initiation message to mobile station M1 tobegin the hard handoff process. Owing to the benign propagationenvironment in the vicinity of mobile station M1, when M1 switches tothe new frequency, it will find the pilots and successfully demodulateforward link traffic from the new Active Set, base stations B2 and B3,as predicted by system S1. M1 determines the hard handoff to besuccessful because the total received pilot exceeds the thresholdMIN_TOT_PILOT. System S1 will deallocate resources previously assignedto communicate with mobile station M1 after it is determined that thehard handoff was successful. This determination may be made by receiptof a message from system S2, or based on a prearranged time duration inwhich no further communication takes place between system S1 and mobilestation M1.

Next, consider mobile station M2, which is in an area of inadequatecoverage by S2 often referred to as a hole. As mobile station M2approaches the S1-S2 border, system S1 predicts that coverage in systemS2 is provided in cell C1. The handoff is initiated in the same manneras described above. However, upon switching to the frequency ofdestination system S2, significant signal energy is not received bymobile station M2 due to the interference caused by obstruction O3. Thatis, total received pilot is less than the threshold MIN_TOT_PILOT. Incurrent systems, this call would be dropped. In the present invention,the mobile station begins recovery techniques.

Once the mobile station determines that the pilot or pilots predicted byS1 are not available, M2 measures the total received power in the newfrequency band and compares it to threshold MIN_RX_PWR. In this example,the only transmitter near M2 is base station B1 and its signal isblocked by obstruction O3, so no significant energy is found in thefrequency band of the destination system. Mobile station M2 thenabandons the handoff and returns to system S1, notifying it that nosystem S2 was found. Assume mobile station M2 continues traveling awayfrom system S1. Since the call was not dropped, as would have been thecase using current methods, a number of options exist. At minimum, thecall can continue on system S1 until it eventually drops because thedistance has become too great. Given that the mobile station environmentis susceptible to change, a second handoff attempt after a delay may besuccessful.

Finally, consider mobile station M3. In like manner to mobile stationsM1 and M2 handoff procedures are initiated with cells C1 and C2 beingthe predicted new Active Set. Due to obstructions O1 and O2, neitherpredicted cell is available to mobile station M3, hence MIN_TOT_PILOT isnot exceeded. Again recovery procedures begin. This time base station BSis within range, however its offset is not in the new Active Set, nor isit transmitting forward link data directed to M3. As such, although thepredicted cells are not available, the minimum received power threshold,MIN_RX_PWR, is exceeded. In the exemplary embodiment of the presentinvention, since the system appears to be available, a search foravailable pilots is performed. When the search is complete, mobilestation M3 returns to system S1 and notifies it of the failed handoffattempt as well as the available pilots, in this case the pilot for cellC5. In the present invention, S1 sends a message to destination systemS2 to set up a forward link on base station BS, then a second attempt athandoff can be made. If the environment has not changed substantially,the second time M3 switches to the new frequency, the call willsuccessfully hand off to base station B5 of destination system S2.

FIG. 3 depicts an exemplary base station. Base station 300 communicateswith other systems (not shown) and with system controller and switch 10,shown in FIG. 1, through system interface 310. Interfrequency handoff isa distributed process, with system controller and switch 10 signalingwith the other switch, and base station 300 handling some of the handoffdetails. System controller 10 determines, in conjunction with basestation 300, that an inter-system hard handoff is necessary. There aremany alternatives for handoff determination as described above,including mobile station location or pilot beacon reception. Thedestination system (not shown) is instructed by the origination systemto begin transmitting forward link traffic on the destination system'sfrequency from a select set of base stations. A database (not shown) incontrol processor 360 may contain the candidate base stations.Alternatively, a suitable list of handoff base station candidates can bereturned from the destination system to control processor 360 throughsystem interface 310. In situations where the destination system is nota CDMA system, other parameters useful for acquiring the destinationsystem can be delivered to control processor 360 through systeminterface 310.

Parameters and instructions from control processor 360 are formed intomessages in message generator 320. Those messages are modulated inmodulator 330 and sent to the mobile station through transmitter 340 andantenna 350. In the exemplary embodiment, modulator 330 is a CDMAmodulator as described in the aforementioned U.S. Pat. Nos. 4,901,307and 5,103,459. In the exemplary embodiment, the list of neighbor basestations, MIN_TOT_PILOT, and MIN_RX_PWR are combined into a singlemessage, referred to herein as the Other Frequency Neighbor List Message(OFNLM). The base station to mobile station message that signals themobile station to begin attempting to acquire the destination systemcontains the destination system Active Set and is called the ExtendedHandoff Direction Message (EHDM). Additional parameters are envisionedthat could be sent to the mobile station to facilitate improved hardhandoff in the event of a handoff attempt failure. For example, aspecific list of offsets to search, a range of offsets to search, or aspecific search algorithm such as-searching offsets in increments of 64chips away from those offsets attempted from those of the base stationslisted in the OFNLM.

Following a failed hard handoff attempt, the mobile station will followthe instructions as given, then return to the original system tocommunicate its findings. Reverse link signals from the mobile stationto base station 300 are received through antenna 390, downconverted inreceiver 380, and demodulated in demodulator 370 under control ofcontrol processor 360.

FIG. 4 depicts an exemplary mobile station 500. Messages arrive atcontrol processor 520 from base station 300 through antenna 610,duplexer 600, receiver 590 and demodulator 570. In the exemplaryembodiment, receiver 590 is a CDMA modulator as described in theaforementioned U.S. Pat. Nos. 4,901,307 and 5,103,459. Upon receipt ofthe EHDM message from base station 300, control processor 520 directsreceiver 590 and transmitter 560 to tune to the frequency of thedestination. At this point, the communication link with the originalsystem has been broken. Control processor 520 directs demodulator 570 toattempt to demodulate pilots at the offsets in the Active Set as givenby base station 300 in the EHDM. The energy in the signals demodulatedwith those offsets is accumulated in pilot energy accumulator 530.Control processor 520 uses the results of the accumulation to compare toMIN_TOT_PILOT. If MIN_TOT_PILOT is exceeded, the handoff is deemedsuccessful. If MIN_TOT_PILOT is not exceeded, recovery operations begin.Alternatively, a requirement of receiving some number N good frames (noCRC errors) within a specific time T can be used to determine if thehandoff attempt is successful.

The first step following an unsuccessful hard handoff attempt is todetermine if the destination system is available. Received energyaccumulator 540 accumulates the total power received in the destinationsystem's frequency band and provides the result to control processor520. Control processor 520 compares those accumulation results with thethreshold MIN_RX_PWR. If MIN_RX_PWR is not exceeded, the handoff attemptis aborted. Receiver 590 and transmitter 560 are retuned to the originalfrequency and control processor 520 generates a message that notifiesbase station 300 that the handoff attempt failed and the destinationsystem was not found to be significantly present. The message isprovided to modulator 550 which modulates the message and provides themodulated signal through transmitter 560, duplexer 600, and antenna 610for transmission.

Mobile station 500 contains system preference information stored insystem preference table 510. If the destination system is not present,mobile station 500 may send alternate system information to base station300, so that mobile station 500 may attempt to acquire a differentsystem on the next hard handoff attempt. For example, a neighboringregion may be covered by multiple systems, which may include acombination of CDMA systems as well as systems of alternatetechnologies. System preference table 510 may be programmed such that ifa first preferred system is not available, acquisition of a secondsystem is attempted. There may be additional systems upon which toattempt handoff, should the second system be unavailable. Handoffattempts can be made in a prioritized order until acquisition has beenattempted on all candidate systems.

If MIN_RX_PWR is exceeded, indicating that the destination system isavailable, mobile station 500 proceeds as previously instructed. In theexemplary embodiment, searcher 580 conducts a search to locate pilotoffsets where base stations in the destination system are available. Toperform a search, searcher 580 generates the PN sequence with a specificoffset. Demodulator 570 correlates the incoming data with the offset PNsequence. Pilot energy accumulator 530 measures the pilot energy forthat offset by accumulating samples for a predetermined time interval.Control processor 520 compares that result to a threshold, called T_ADD,to determine whether a pilot is available for that offset. Searcher 580then moves to the next offset candidate. The process repeats until thereare no more candidate offsets to measure. The search operation processis described in detail in U.S. Pat. No. 5,805,648 entitled “METHOD ANDAPPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATIONSYSTEM”, issued Sep. 8, 1998, which is assigned to the assignee of thepresent invention and incorporated by reference herein. Alternate searchalgorithms can be substituted in searcher 580 without modification tothe present invention.

The search subsequent to the hard handoff failure may be performed overall possible offsets or a subset thereof. For example, a range ofoffsets may be searched. In the exemplary embodiment, the OFNLM containsthe subset of offsets to be searched. In the exemplary system,neighboring base stations are separated by integer multiples of 64chips. If one base station offset in the system is known (even if it isnot currently available), only offsets that are integer multiples of 64from that known offset need to be searched in order to attemptacquisition on the complete set of neighbor base stations. A combinationof spaced offsets in a specific range or number of ranges can also besearched.

When the destination system is an alternate technology, there may bedifferent procedures to perform which will yield information that willimprove subsequent hard handoff attempts. For example, when thedestination system is TDMA, the mobile station may measure the in bandenergy at a plurality of frequency subbands and report this informationto the origination system. Or in the case of a neighboring AMPS system,the base station can send an OFNLM specifying frequencies for the analogcontrol channels. However, it may not be necessary to send thefrequencies of the control channels if they are already known. In thatcase, if the mobile station finds the voice channel to which it washanded off is too weak, the mobile station can proceed to measure thereceived power on the analog control channels. It may also determine thedigital color code (DCC) for the control channel. The DCCs providebetter determination of the cell in case the mobile station might beable to receive multiple cells in an area. The frequencies and DCCs ofthe strongest analog base stations can be returned as information toassist with a subsequent handoff attempt. Further discussion of the useof DCCs can be found in chapter 3 of “Mobile Cellular TelecommunicationsSystems” by William C. Y. Lee.

After mobile station 500 completes the requisite tasks, receiver 590 andtransmitter 560 are retuned to the original frequency and controlprocessor 520 notifies base station 300 through modulator 550,transmitter 560, duplexer 600, and antenna 610 that the handoff attemptfailed and delivers any information that has been discovered duringsubsequent system search procedures.

The flow chart in FIG. 5 illustrates operation of the preferredembodiment of this invention. After determining that a handoff isimminent, the origination system predicts the list of neighbor basestations on the neighboring system's frequency in box 50. Proceed to 52,a base station in the origination system sends to the mobile station theOther Frequency Neighbor List Message (OFNLM) described above. In block53, the Active Set for the new frequency is determined. In block 54, thedestination system sets up the forward link as specified in the ExtendedHandoff Direction Message (EHDM). In block 56, the base station in theorigination system sends the Extended Handoff Direction Message (EHDM)to the mobile station to initiate the inter-frequency hard handoff.Following that message, in 58, the mobile station tunes to the newfrequency and attempts to acquire the destination system according tothe Active Set information in the EHDM message.

In block 60, the mobile station measures the pilot energy, the sum ofthe energy of all pilots in the Active Set, and if the received totalpilot energy exceeds that of parameter MIN_TOT_PILOT, proceed to 62, asuccessful hard handoff has occurred. The exemplary embodiment envisionsthat a mobile station is capable of being handed off directly into asoft handoff condition in the destination system, although that is not arequirement. A single pilot in the new Active Set whose received pilotenergy exceeds that of parameter MIN_TOT_PILOT is sufficient for asuccessful handoff.

From 60, if MIN_TOT_PILOT is not exceeded, proceed to 68. In 68, iftotal received power in the frequency band exceeds parameter MIN_RX_PWRindicating the general presence of the destination system, proceed to66, otherwise go to 69.

An alternative embodiment would be to check total received power beforepilot energy. If the MIN_RX_PWR threshold is not exceeded, the handoffis aborted. This may be faster in some implementations.

In 66, search the possible offsets for available pilot signals. Anyalternate search strategy can be performed here as well. When search iscomplete, proceed to 65. The mobile station returns to the originalsystem in 65, then proceeds to 64. In 64, make necessary changes toOFNLM and return to 52, where the operation proceeds as described above.

In 69, the mobile station returns to the original system, then proceedsto 72. From 72, the decision can be made to continue attempting handoffby proceeding to 70, or the handoff procedure can be aborted byproceeding to 74. An optional delay is introduced in 70, then proceed to64.

In an alternative embodiment of the present invention, the base stationsends the mobile station an extended list of base stations which may beavailable at the point in which the mobile station is entering thedestination system. In this alternative embodiment, no forward links areimmediately set up in the destination system. Rather the mobile stationsimply determines whether the strength of any of the signals provided byany of the extended list of candidate systems are adequate to support acommunication link. The mobile station monitors the forward link signalsof each of the base stations in the extended list of candidate basestations.

After monitoring the signal strength of each of base stations in theextended list of candidate base stations, the mobile station necessarilyreturns to the original system and sends a message indicating the signalstrength of the forward links of the candidate base stations. In theexemplary embodiment, the mobile station compares the strength of thesignals received by each of the base stations in the extended list to apredetermined threshold T_ADD and reports only whether the measuredsignal power is above or below the threshold.

The base station of the original system receives the informationregarding the signal strength of each of the base stations in thedestination system and from this information the base station of theoriginal system generates an Active Set list. This list is provided tothe destination system which sets up forward links for the mobilestation in accordance with the Active Set list provided by the originalsystem. The base station of the original system transmits the activelist to the mobile station which attempts to acquire the base stationsin the active list and, if acquisition is successful, transmission tothe mobile station is available without interruption.

Referring to FIG. 2, the alternative embodiment will be described interms of acquisition of mobile M3. When original system S1 determinesthat mobile M3 should begin hard hand off operations to destinationsystem S2, the base station in original system S1 which is currently incommunication with mobile station M3 generates an extended list of basestations in S2 which the mobile station may be able to acquire. In theexemplary embodiment, the extended candidate list would likely consistsof the parameters necessary to perform a search on all of base stationsB1, B2, B3, B4 and B5, as well as additional base stations indestination system S2 (not shown). Note in the alternative embodiment,no information regarding M3 has, as of yet, been provided to thedestination system S2.

Mobile station M3 tunes to the frequency of destination system S2 andmeasures the energy on each of the pilot channels of the base stationsin the extended candidate list. In the example of mobile station M3, themobile station would transmit back a message to the base station on theoriginal system S1 a message indicating that acquisition upon basestation B5 was possible. In response to this message, the base stationin the original system would generate an Active Set list consistingsolely of base station BS.

The base station in the original system would send a message to thedestination system S2, indicating that a forward link for the mobilestation M3 should be provide on base station B5. In response to thismessage, the destination system S2 sets up a forward link for mobilestation M3 on base station B5. The Active Set list is sent to mobilestation M3. In response to the Active Set message, mobile station M3attempts acquisition of base station B5.

Referring to FIG. 3, base station 300 of the original system generatesan extended candidate list in message generator 320 and provides themessage to modulator 330. The message is modulated by modulator 330 andprovided to transmitter 340 which upconverts and amplifies the signaland transmits the resulting signal through antenna 350.

Referring to FIG. 4, the transmitted signal is received by mobilestation 500 by antenna 610 and is downconverted, filtered and amplifiedby receiver 590. The received signal is then demodulated by demodulator570 and provided to control processor 520. Control processor 520 thengenerates a set of commands directing a search to be performed bysearcher 580. Searcher 580 provides a set of search demodulationparameters to demodulator 570. The demodulated signals are provided topilot energy accumulator 530 which measures the strength of the pilotsof the base stations of the extended candidate list. The energy of eachof these candidates is provided to control processor 520 which comparesthe measured energy with a threshold T_ADD. Control processor 520generates a message which signifies which, if any, of the candidate basestation's signals exceed the threshold.

The message is provided to modulator 550 where it is modulated. Themodulated signal is then provided to transmitter 560 where it isupconverted, amplified, and transmitted through antenna 610.

Referring back to FIG. 3, the message indicating the strengths of thecandidate base stations is received by antenna 390 of base station 300of the original system. The signal is downconverted and amplified byreceiver 380 and provided to demodulator 370. Demodulator 370demodulates the signal and provides the result to control processor 360.Control processor 360 generates an Active Set list for the destinationsystem in accordance with the information in the message transmitted bymobile station 500 indicating the results of its search. In theexemplary embodiment, the Active Set list will consist of all basestations whose signals, when monitored by mobile station 500, exceededthe energy threshold T_ADD.

Control processor 360 sends the Active Set list to system interface 310which sends a message indicating the Active Set list to the destinationsystem S2. Capacity issues allowing, destination system S2 providesforward link channels on each of the systems in the Active Set list.

Control processor 360 also provides the Active Set list to messagegenerator 320. The resulting message is modulated by modulator 330 andtransmitted as described above.

Mobile station 500 receives the message by antenna 610, demodulates thesignal as described above, and provides the message to control processor520. Control processor 520 then provides information regarding theActive Set list to demodulator 570 and receiver 590 and a hard handoffto the destination system S2 is attempted using the parameters of thebase stations in the Active Set list. It should be noted that because,in this example, the active list was determined by mobile station 500,the mobile station need not receive the Active Set list, since it knowsthe station on the list apriori. Thus, in an alternative, embodiment,the mobile station may delay a predetermined time period and performhandoff to the base stations who's signals exceeded the threshold. If,on the other hand, the Active Set is not simply a copy of the basestations which exceeded the threshold but rather also takes into accountparameters unknown to the mobile station, such as capacity parameters ofS2, then transmission of the message would prove of value.

In a variation on the above described alternative embodiment, the mobilestation periodically tunes to the new frequency and measures the offsetssupplied in the OFNLM without direction from the base station. Theperiod may be specified in the OFNLM. After the search is complete, themobile station returns to the origination system and reports itsfindings. This information gained by polling the neighboring system canbe used to determine the Active Set for a subsequent handoff attempt, aswell as to assist in determining whether to initiate a handoff to thatsystem.

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

We claim:
 1. In a wireless communication system wherein a mobile stationis moving from the coverage area of an origination system operating at afirst frequency into the coverage area of a destination system operatingat a second frequency, a method for providing handoff from saidorigination system to said destination system, the method comprising:predicting at least one PN offset associated with the destinationsystem; transmitting, at the first frequency, from said originationsystem to said mobile station a set of search parameter data associatedwith the at least one PN offset; determining at said mobile station theavailability of said destination system in accordance with said set ofsearch parameter data; transmitting from said mobile station to saidorigination system a message indicative of the received strength ofsignals associated with the at least one PN offset measured at themobile station; generating at said origination system a set ofacquisition parameters in accordance with said message from said mobilestation; and attempting at said mobile station to acquire saiddestination system in accordance with said set of acquisitionparameters.
 2. The method of claim 1, wherein said generating furthercomprises electing at said origination system a new set of at least onePN offset associated with the destination system.
 3. The method of claim1, wherein said attempting further comprises electing at said mobilestation a new set of at least one PN offset associated with thedestination system.
 4. A method of providing handoff of a mobile stationfrom an origination system operating at a first frequency to adestination system operating at a second frequency, comprising:receiving from the origination system at least one PN offset associatedwith the destination system and a minimum total received pilot value;tuning, at the mobile station, to the second frequency; determining apilot energy associated with the at least one PN offset received at themobile station; comparing the determined pilot energy with the minimumtotal received pilot value; and sending, from the mobile station to theorigination system, a message based on said comparing.
 5. The method ofclaim 4 wherein said message comprises said determined pilot energy. 6.The method of claim 4 wherein said receiving further comprises receivingfrom the origination system a set of additional parameters comprisinginstructions to the mobile station following a failed hard handoffattempt.
 7. The method of claim 6 wherein said sending is based on saidinstructions.
 8. The method of claim 4 further comprising performing asearch at the second frequency based on said comparing.
 9. The method ofclaim 8 wherein said performing a search is based on said instructions.10. A method of providing handoff of a mobile station from anorigination system operating at a first frequency to a destinationsystem operating at a second frequency, comprising: receiving from theorigination system at least one PN offset associated with thedestination system and a received power threshold; tuning, at the mobilestation, to the second frequency; determining a pilot energy associatedwith the at least one PN offset received at the mobile station;measuring in-band energy received by the mobile station at the secondfrequency; comparing the measured in-band energy with the received powerthreshold; and sending, from the mobile station to the originationsystem, a message based on said comparing.
 11. The method of claim 10wherein said message comprises said measured in-band energy.
 12. Themethod of claim 10 wherein said receiving further comprises receivingfrom the origination system a set of additional parameters comprisinginstructions to the mobile station following a failed hard handoffattempt.
 13. The method of claim 12 wherein said sending is based onsaid instructions.
 14. The method of claim 10 further comprisingperforming a search at the second frequency based on said comparing. 15.The method of claim 14 wherein said performing a search is based on saidinstructions.
 16. A method of providing handoff of a mobile station froman origination system operating at a first frequency to a destinationsystem operating at a second frequency, comprising: sending from theorigination system to the mobile station at least one PN offsetassociated with the destination system and a minimum total receivedpilot value; sending from the origination system to the mobile station amessage directing the mobile station to perform a hard handoff to thedestination system; receiving at the origination system from the mobilestation a message based on a total received pilot value measured by themobile station at the second frequency; and sending to the mobilestation at least one adjusted PN offset having a different value thansaid at least one PN offset based on said message.
 17. The method ofclaim 16 wherein said sending to the mobile station further comprisessending to the mobile station a set of additional parameters comprisinginstructions to the mobile station following a failed hard handoffattempt.
 18. The method of claim 16 wherein said message furthercomprises measurements from a search performed at the second frequency.19. A method of providing handoff of a mobile station from anorigination system operating at a first frequency to a destinationsystem operating at a second frequency, comprising: sending from theorigination system to the mobile station at least one PN offsetassociated with the destination system and a received power threshold;sending from the origination system to the mobile station a messagedirecting the mobile station to perform a hard handoff to thedestination system; receiving at the origination system from the mobilestation a message based on a received power measured by the mobilestation at the second frequency; and sending to the mobile station atleast one adjusted PN offset having a different value than said at leastone PN offset based on said message.
 20. The method of claim 19 whereinsaid sending to the mobile station further comprises sending to themobile station a set of additional parameters comprising instructions tothe mobile station following a failed hard handoff attempt.
 21. Themethod of claim 19 wherein said message further comprises measurementsfrom a search performed at the second frequency.
 22. A mobile stationapparatus comprising: a receiver configured to tune to one of at leasttwo system frequencies based on a frequency control signal; and acontrol processor configured to generate the frequency control signaldirecting said receiver to tune to a first frequency associated with anorigination system, to receive from the origination system at a firstfrequency at least one PN offset associated with a destination systemand a minimum total received pilot value, to direct said receiver totune to a second frequency associated with the destination system, tocompare a pilot energy measured at the second frequency with the minimumtotal received pilot value to generate a comparison, to direct saidreceiver to tune back to the first frequency based on the comparison,and to generate a message for the origination system based on thecomparison.
 23. The mobile station apparatus of claim 22 wherein saidmessage comprises the pilot energy measured at the second frequency. 24.The mobile station apparatus of claim 22 wherein said control processoris further configured to receive from the origination system a set ofadditional parameters comprising instructions to the mobile stationfollowing a failed hard handoff attempt.
 25. The mobile stationapparatus of claim 24 further comprising a searcher configured toperform a search at the second frequency based on commands from saidcontrol processor, and wherein said control processor directs saidsearcher to perform the search based on the instructions.
 26. A mobilestation apparatus comprising: a receiver configured to tune to one of atleast two system frequencies based on a frequency control signal; and acontrol processor configured to generate the frequency control signaldirecting said receiver to tune to a first frequency associated with anorigination system, to receive from the origination system at a firstfrequency at least one PN offset associated with a destination systemand a received power threshold, to direct said receiver to tune to asecond frequency associated with the destination system, to compare areceived power measured at the second frequency with the received powerthreshold to generate a comparison, to direct said receiver to tune backto the first frequency based on the comparison, and to generate amessage for the origination system based on the comparison.
 27. Themobile station apparatus of claim 26 wherein said message comprises thereceived power measured at the second frequency.
 28. The mobile stationapparatus of claim 26 wherein said control processor is furtherconfigured to receive from the origination system a set of additionalparameters comprising instructions to the mobile station following afailed hard handoff attempt.
 29. The mobile station apparatus of claim28 further comprising a searcher configured to perform a search at thesecond frequency based on commands from said control processor, andwherein said control processor directs said searcher to perform thesearch based on the instructions.